Recently, the use of electronic still cameras and video cameras employing a CCD or a solid-state image pickup device has been expanding rapidly, and the needs for miniaturization and weight/cost savings are increasing. Miniaturization and weight/cost savings are also needed for photographing lenses that are built into cameras.
The optical system using a solid-state image pickup device employs a crystal filter so as to prevent moiré caused by the periodical structure of the image pickup device. Therefore, the thickness and location of the crystal filter must be taken into consideration in designing optical systems. In particular, the telecentricity of an incident ray to the image side to face an image forming plane is an important factor of the design.
Thus the photographing lens for cameras using a solid-state image pickup device must have good telecentricity and a long retrofocal length for the use of the crystal filter, as well as high resolution.
The photographing lens is also required to have a low F-number for entrance of a larger amount of light so as to compensate for the deterioration of sensitivity caused by the high pixel value of the solid-state image pickup device, and its compactness is needed to realize miniaturization and weight savings.
To satisfy these requirements, lens-designing technologies for adequately arranging the refractive powers of the component lenses of the photographing lens and precise technologies for realizing good performance are necessary.
The conventional optical systems using a solid-state image pickup device are as follows:
1) Japanese Patent Application Laid-Open No. Pyung 9-297264; and
2) Japanese Patent Application Laid-Open No. Pyung 10-293246.
The optical systems disclosed in the cited patents 1) and 2) are retrofocus optical systems comprising, in order from the object side, a lens group having a negative refractive power, an iris diaphragm, and a second lens group having a positive refractive power. These conventional optical systems may easily secure a large amount of peripheral light and have good telecentricity, but the correction of aberration, particularly distortion, is difficult to achieve, and the overall length of the optical systems is increased.